This invention relates to fire alarm systems, and, more particularly, to fire alarm systems that provide guidance for building dwellers to flee a building that is burning.
Fire alarms are generally triggered by manual emergency switches, heat sensors, or smoke sensors. Upon being alerted by a fire alarm, the occupants of a building must instantaneously take stock of their situations and decide if they should flee the building, and then what path they should take to evacuate. Although many persons cannot function well in such a sudden crisis environment, the situation is made more problematic by the physical conditions that occur during a building fire. Very often smoke from burning carpets and furniture is so dense and noxious that common visual landmarks become obscured, and individuals become disoriented and panicked. In these circumstances many individuals may not find the best path to the nearest exit door or window, even in surroundings that are otherwise familiar. It is known that most fire victims succumb to smoke, not to the heat of the fire.
There are devices known in the prior art that are intended to direct occupants in the event of an emergency such as a fire. Alarm systems in commercial and industrial buildings provide illuminated exit signs to indicate all designated exit doors, and the signs are typically connected to auxiliary power to continue operation in the event of main circuit power failure. Most municipal codes require that such signs be mounted above the door or window. However, most smoky fires proceed by first building up a smoke layer adjacent to the ceiling of a room or hall, and that smoke layer grows rapidly by expanding downwardly from the ceiling. The diffuse light of an exit sign is easily scattered or absorbed by the smoke. Thus heavy smoke may obscure the typical exit sign at a fairly early stage in the development of a blaze, even while survivors may move about close to the floor and seek escape.
Another apparatus in the prior art, described in U.S. Pat. No. 5,572,183, employs a rotating mirror to direct indicator light to a plurality of fiberoptic light guides that extend to sequentially spaced points along a ceiling of a room or hall. Each fiberoptic guide projects a path indicating image, and the rotating mirror causes a plurality of indicator images to be projected sequentially in a spaced apart, progressive manner along the floor of a room or hall. This same technique is taught using a plurality of laser diodes, each installed at one of the sequentially spaced points, and each projecting a path indicating image. These installations require extensive wiring or fiberoptic installation, as well as persistent maintenance to assure operability and alignment of all the high precision components of the system, and cannot be considered practical in commercial or mechanical terms.
Thus, although it is known to project a path indicating image using a laser diode as a light source, and that this light source is superior in penetrating the smoke accumulation that accompanies fire, there is no practical system for taking advantage of this attribute.
There are also known in the prior art alarm systems that provide secondary alarm indications in response to the audible signal of a primary alarm, such as a smoke detector. U.S. Pat. No. 5,177,461 describes an apparatus for attracting fire fighters and rescuers to the exterior of a building in which a smoke alarm has been triggered. However, there is no suggestion in the prior art of the provision of an alarm system that utilizes the audible alert of a smoke alarm as a trigger to actuate an escape path indicating system for the occupants of the burning building.
Moreover, it has been observed that the audible alarm signal of a smoke alarm or the like may form standing waves in a room or hall, particularly in the confined spaces of a typical alarm device mounting position, such as a wall corner or ceiling corner. A standing wave causes zones of compression and rarefaction to stagnate in stable positions. These zones are alternately spaced on a scale that approximates one-half wavelength of the alarm tone. Given an alarm tone in the range of 2.5 KHz, the half-wavelength distance is approximately 2.5 inches (6.25 cm). A typical prior art secondary alarm system, such as the patented one referenced above, uses a single microphone to detect the primary audible alarm. It is clear that a small change in the mounting position of the device can substantially affect the reception of the primary alarm audible signal, and that the standing wave problem may significantly impact the installation of the prior art device.
Even if a secondary alarm system is properly placed, it must continually pick up ambient sound and noise and constantly evaluate this signal for the presence of the primary alarm audible signal. Operating an acoustic pickup (microphone) and processing the signal for detection purposes generally requires the use of several active systems that ceaselessly draw electrical power. As a result, power requirements for these devices demand either large and heavy batteries, or frequent replacement of smaller batteries. Large and heavy batteries necessitate a housing sufficiently large to support them, resulting in a device that is cumbersome and too weighty for safe mounting on a ceiling or wall. On the other hand, the need to replace smaller batteries more frequently creates a high maintenance demand that many homeowners are not well-disposed to meet. Neither option is attractive for a successful product design.